Pulse Oximeter Principles
C-2
www.zoll.com
9650-002360-01 Rev. A
The Masimo SET MS board pulse oximeter assumes that arterio-venous shunting is highly
variable and that fluctuating absorbance by venous blood is the major component of noise
during the pulse. The MS board decomposes S(660) and S(905) into an arterial signal plus a
noise component and calculates the ratio of the arterial signals without the noise:
S(660) = S1 + N1
S(905) = S2 + N2
R = S1/S2
Again, R is the ratio of two arterial pulse-added absorbance signals and its value is used to find
the saturation SpO
2
in an empirically derived equation into the oximeter’s software. The values
in the empirically derived equation are based upon human blood studies against a laboratory
co-oximeter on healthy adult volunteers in induced hypoxia studies.
The above equations are combined and a noise reference (N’) is determined:
N’ = S(660) - S(905) x R
If there is no noise, N’ = 0: then S(660) = S(905) x R, which is the same relationship for
traditional pulse oximeter.
The equation for the noise reference is based on the value of R, the value being sought to
determine the SpO
2
. The MS board software sweeps through possible values of R that
correspond to SpO
2
values between 1% and 100% and generates an N’ value for each of these
R-values. The S(660) and S(905) signals are processed with each possible N’ noise reference
through an adaptive correlation canceler (ACC), which yields an output power for each
possible value of R (i.e., each possible SpO
2
from 1% to 100%). The result is a Discrete
Saturation Transform (DST
™
) plot of relative output power versus possible SpO
2
value as
shown in the following figure where R corresponds to SpO
2
= 97%:
Figure C-1 Pulse Oximeter Discrete Saturation Transformation
The DST plot has two peaks: the peak corresponding to the higher saturation is selected as the
SpO
2
value. This entire sequence is repeated once every two seconds on the most recent four
seconds of raw data. The MS board SpO
2
therefore corresponds to a running average of arterial
hemoglobin that is updated every two seconds.
